1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine which supplies a high voltage to a spark plug for each engine cylinder, and it also relates to a method of manufacturing the same.
2. Description of the Related Art
FIG. 8 is a plan view of a known ignition apparatus 1 for an internal combustion engine shown in Japanese Patent Application Laid-Open No. Hei 7-29751. FIG. 9 is a cross sectional view of the ignition apparatus 1 for an internal combustion engine taken along line XIV—XIV in FIG. 8. FIG. 10 is an electric circuit diagram of the ignition apparatus 1 for an internal combustion engine shown in FIG. 8.
The ignition apparatus 1 illustrated is used by a simultaneous ignition system for an internal combustion engine, that is, the opposite ends of a secondary coil 6 of each transformer are finally connected to two spark plugs. The ignition apparatus 1 is provided with three transformers 5A, 5B, 5C, and is used for an internal combustion engine having six cylinders.
The first through third transformers 5A, 5B, 5C of the ignition apparatus 1 are built into a casing 4 that is molded with a resin and has a plurality of high-voltage towers 3 formed at its head. Each of the first through third transformers 5A, 5B, 5C includes a closed magnetic circuit core 6 which forms a hollow-rectangular-shaped closed magnetic circuit, a primary coil 8 which has a conducting wire wound around a primary bobbin 7 surrounding the closed magnetic circuit core 6, and a secondary coil 10 which has a conducting wire wound around a secondary bobbin 9 surrounding the primary coil 8. In addition, a switching module 12, which is a circuit unit for controlling the energization of the primary coils 8, is built into the casing 4.
The conducting wire of each primary coil 8 is finally connected at its one end through a first conductor 13 to a power supply (not shown) such as the battery of a vehicle. The conducting wire of each primary coil 8 is also connected at its other end through a second conductor 14 with a terminal of a corresponding switching module 12, e.g., a collector of a corresponding power transistor which constitute the switching module 12. Moreover, a third conductor 15 in the form of a power supply ground wire and fourth conductors 16 in the form of signal lines connected with a control unit (not shown) are respectively connected with the switching module 12.
Also, the conducting wire of each secondary coil 10 is connected at its opposite ends with the corresponding high-voltage tower terminals 17 in the high-voltage towers 3, respectively. The high-voltage tower terminals 17 are electrically connected with spark plugs (not shown) through high-tension cords (not shown).
The first conductor 13, the second conductor 14, the third conductor 15 and the fourth conductors 16 are integrally formed with one another through a resin to provide a wiring module 18.
The wiring module 18, the switching module 12, and the first through third transformers 5A, 5B, 5C are fixedly attached to the inner surface of the casing 4 through an insulating resin portion 11 made of a thermosetting resin such as an epoxy resin.
Now, the operation of the ignition apparatus 1 for an internal combustion engine as constructed above will be explained below.
The switching module 12 is driven by control signals from the control unit of the internal combustion engine, so that a primary current flowing through each primary coil 8 is controlled to be supplied and interrupted in an appropriate manner by the switching module 12.
When the primary current to the primary coils 8 is interrupted or cut off at prescribed ignition timing of the internal combustion engine, counterelectromotive forces are generated in the primary coils 8 whereby a high voltage is produced in the secondary coils 10 of the transformers 5A, 5B, 5C. As a result, the high voltage thus produced is impressed on the spark plugs (not shown) connected with the secondary coils 10, whereby air fuel mixtures in the unillustrated engine cylinders are dielectrically broken down, and fired by sparking of the corresponding spark plugs caused by discharging of secondary current flowing through the respective secondary coils 10.
With the known ignition apparatus for an internal combustion engine, the wiring module 18 is formed by integrating the first conductor 13, the second conductor 14, the third conductor 15 and the fourth conductors 16 with one another through a resin. However, as the number of the transformers 5A, 5B, 5C increases, it is necessary to accordingly increase the number of the conductors 13 through 16, thus making the wiring paths of the respective conductors 13 through 16 further complicated as well as enlarging the size of the wiring module 18 itself.
In addition, if the number of the transformers is increased so much, it sometimes becomes impossible to arrange the wiring module in one and the same plane, and it is required to construct the wiring module by means of a plurality of stacked or laminated wiring module components.
Thus, the wiring module 18, which is formed by integrating all the conductors including the first conductor 13, the second conductor 14, the third conductor 15 and the fourth conductors 16 with one another through a resin as described above, gives rise to the following problem. That is, the degree of freedom of the wiring paths of the wiring module 18 is low, and hence if the size of the wiring module 18 is accordingly increased, or if the number of module components of the wiring module 18 is accordingly increased, the size of casing 4 receiving therein the wiring module 18 would have to be increased, thus making it difficult to install the ignition apparatus 1 for an internal combustion engine in the limited engine room of a vehicle.